Immunoglobulin class switching is regulated by the action of lymphokines. Of these the best studied example is IL-4 regulation of production of both IgG and IgE in vitro and of IgE in vivo. IgE production in both primary and secondary responses to conventional antigens and to infections with parasitic nematodes are inhibited by treatment with antibodies to IL-4 or to the IL-4 receptor. Indeed, in mice infected with Heligmosomoides polygyrus, anti-IL-4 receptor antibody not only blocks IgE responses but also blocks the acquisition of protective immunity. In order to study the process of Ig class switching more precisely, an assay for the DNA deletional event that is involved in switching was developed. This assay detects the appearance of the chimeric switch (S) regions that are characteristic of switched cells. The assay depends upon digestion with restriction enzymes, such as EcoRI, the formation of circular DNAs by ligation and PCR amplification across the ligation joint. With primers that recognize sequences 5'to Smu and 3' to SGammal, such amplification can only occur in cells that have undergone switching to gammal expression. Methods to quantitate this assay have been developed and it has been used to study the precise timing of the DNA deletional events, in normal cells, that occur in switching. One of the key events that precedes class switching is appearance of a transcript initiated 5' of the S region of the isotype to which the cell will switch. These transcripts are referred to as germ line transcripts and the transcribed exon is designated I. A phosphorothioate anti-sense oligonucleotide complementary to a sequence in IGamma2b proved to a potent stimulant of the expression of the IGamma2b transcript, a powerful stimulant of B cell DNA synthesis and an inhibitor of immunoglobulin secretion. Efforts to understand the mechanism of action of this oligonucleotide are in progress.